In the case of electrically commutated motors, in particular d.c. motors, electrochemical, electrical and/or mechanical processes may result in wear on the carbon brushes of the commutator. In extreme cases, this wear may cause the electrically commutated motor to fail, since, in the case of corresponding critical abrasion of the carbon brushes, current is no longer able to be conducted through the armature windings of the electric motor.
In particular, if an electrically commutated d.c. motor of this type is used in a brake system, for example as the pump motor, critical wear on or abrasion of the carbon brushes must be detected at an early stage. A variety of methods may be used to detect wear of this type in good time, for example in an electrohydraulic brake system, as described in German Patent Application No. DE 199 23 689. For example, it is possible to estimate the amount of wear on the carbon brush in a pump motor of the hydraulic portion of the brake system on the basis of the number of brakings during vehicle operation.
A further method and a device, respectively, for detecting the amount of wear on the drive system of an electric motor are known from the non-pre-published German Patent Application No. 10 2004 035318.
In this case, the abrasion of the carbon brushes of an electric motor is detected as wear using a wear variable. This wear variable is determined as a function of a speed variable representing the rotational speed of the electric motor. With the help of this speed variable, a measure of the number of revolutions completed by the electric motor is derived. Assuming a corresponding amount of wear, the abrasion of the carbon brushes is thereby detectable.